Add support for nullable arrays with infer_schema!.

[Aaronepower]

Currently infer_schema! generates the this schema from this psql. Which is incorrect since the elements of the array could be null.

Sql

CREATE TABLE posts (
  id SERIAL PRIMARY KEY,
  title VARCHAR NOT NULL,
  body timestamptz[7] NOT NULL,
  published BOOLEAN NOT NULL DEFAULT 'f'
)

Generated schema

table! {
    posts (id) {
        id -> Int4,
        title -> Varchar,
        body -> Array<Timestamptz>,
        published -> Bool,
    }
}

[sgrif]

To clarify, I want to continue to have the generated type be Array<Timestamptz>, but make sure that deserializing to Vec<Option<NaiveDateTime>> deserializes successfully. Even though considering it to be Array<Timestamptz> is technically incorrect, as it could in theory have a null in there, I want to provide some flexibility here. There is no way to represent whether the elements of an array can or cannot be null in postgres.